(1) Field of the Invention
This invention relates to an inkjet printing apparatus for printing on printing paper by discharging ink droplets from inkjet heads while moving the inkjet heads and the printing paper relative to each other.
(2) Description of the Related Art
There are inkjet printing apparatus which form images on printing paper while moving the inkjet heads and the printing paper relative to each other. Such inkjet printing apparatus are operable in a multi-pass mode for performing printing while moving the inkjet heads in a width direction of the printing paper, or a one-pass mode for performing printing with the inkjet heads covering the width of the printing paper, and thus without moving the inkjet heads in the width direction of the printing paper.
In the apparatus of the one-pass mode, the inkjet heads covering printing areas of the printing paper are arranged on an inkjet head holder, and the inkjet heads usually are longer than those of the apparatus of the multi-pass mode. Further, an apparatus is known which has a printing position and a cap position located above the printing paper in order to avoid a large occupancy area. The printing position is a position where the printing paper and the inkjet heads are close to each other. The cap position is a position where the printing paper and the inkjet heads are away from each other and the inkjet heads are closed by caps. For this purpose, a construction is needed for raising and lowering the inkjet heads together with the inkjet head holder relative to the printing paper.
A known construction for raising and lowering the inkjet head holder includes, for example, one pulse motor, and a transmission mechanism for transmitting torque to opposite ends of the inkjet head holder.
There is an infusion pump as a known device for performing the above raising and lowering operations using one pulse motor. See Japanese patent No. 3320179 (paragraphs “0077” and “0078”, and FIGS. 5-8), for example.
This infusion pump includes a slit disk attached to a shaft of the pulse motor and having slits arranged at unequal intervals in a circle, with mutually different lengths in a direction of rotation, a table storing values of at least either of the lengths in the direction of rotation of the slits and the slit intervals, a photosensor for detecting the slits of the slit disk and outputting detection signals, and a rotational position detecting device for detecting a rotational position of the pulse motor by identifying, based on the detection signals of the photosensor and the table, at least one of a slit in the position of the photosensor and a slit interval. This construction can detect a reversal of the pulse motor.
The conventional example with such a construction has the following drawback.
The inkjet heads for the one-pass mode, in particular, are long in the width direction of the printing paper, and with the construction which transmits the torque of one pulse motor through the transmission mechanism, the transmission mechanism becomes too long sideways, resulting in a notable rotational lag between right and left. Therefore, the conventional apparatus noted above cannot appropriately raise and lower the inkjet head holder holding the inkjet heads.
Then, it is conceivable to provide two pulse motors for the opposite ends in the width direction of the inkjet head holder, respectively, to raise and lower the inkjet head holder while synchronizing the two pulse motors. However, with the plurality of controlled objects, there occurs a problem that it is difficult to perform raising and lowering control of the inkjet head holder appropriately while detecting a reversal and stepout of each pulse motor. Particularly when the opposite ends of the inkjet head holder move to different height positions, the inkjet head holder will be subjected to torsion. When a raising and lowering operation is continued in such state, in the worst case the inkjet head holder can be damaged.
The same problem as above may occur not only with an apparatus for the one-pass mode, but with an apparatus for the multi-pass mode, which may be an inkjet printing apparatus having a mechanism for raising and lowering the inkjet head holder with a plurality of motors.